Tachometer Intermittent Failure with MIL On While Driving
2026/06/05
Fault Phenomenon
Vehicle Info: 2012 Buick Excelle sedan equipped with LDE 1.6L engine.
Customer Feedback:
While driving, the engine tachometer sometimes stops working, and the engine malfunction indicator lamp illuminates intermittently, yet the vehicle runs normally.
If the engine is shut off and restarted under this condition, the starter operates normally, but the engine fails to start. It can only be ignited after waiting several minutes for the engine to cool down.
Fault Diagnosis
Maintenance technicians carried out inspection with a diagnostic scanner and retrieved a fault code stored in the Engine Control Module (ECM): "P0335-00 Crankshaft Position Sensor Circuit".
After clearing the fault code and conducting a new road test with repeated scanning, we found no fault codes when the engine was cold. The fault only occurred intermittently when the engine reached operating temperature, and the retrieved fault code remained unchanged every time the malfunction appeared.
Based on the fault code indicated by the engine system, maintenance technicians carried out the following analysis:
The crankshaft position sensor circuit consists of three branches: a 5V reference voltage circuit supplied by the Engine Control Module (ECM), a low reference voltage circuit, and an output signal circuit.
The crankshaft position sensor generates a variable-frequency on/off DC voltage signal. The ECM uses each pulse of the crankshaft position signal to calculate engine crank speed and identify the exact crankshaft position.
Fault code P0335 will be logged if any of the following malfunctions occur:
1.The Engine Control Module (ECM) fails to detect crankshaft position sensor pulses for more than 4 seconds.
2.The Engine Control Module (ECM) fails to detect crankshaft position sensor pulses for more than 0.1 seconds.
3.The ECM misses crankshaft position sensor pulses during 2 out of 10 full engine crankshaft rotations.
If the crankshaft position sensor malfunctions, the engine tachometer will stop working. The ECM will then rely on the camshaft position sensor to calculate engine speed and crankshaft position.
At this point, the troubleshooting logic became clear. Referring to the relevant circuit diagram, technicians performed a series of tests on the crankshaft position sensor circuit step by step:
1.Turn off the ignition switch and disconnect the crankshaft position sensor connector.
2.Measure the resistance between Terminal 2 and vehicle ground; the reading is less than 5Ω, which confirms the sensor ground circuit is normal.
3.Switch the ignition to ON position, measure the voltage between the 5V reference voltage terminal and ground, reading 5.07V. The sensor 5V power supply circuit functions normally.
4.Measure the voltage between the signal circuit terminal and ground, reading 5.02V, indicating the signal wire is intact.
Reconnect the wiring harness connector of the crankshaft position sensor, start the engine and let it idle. The measured sensor supply voltage is 5.07V, and the signal wire voltage reads 2.7V, both within normal range.
After comprehensive circuit inspection, no electrical faults were detected. Technicians then replaced both the crankshaft position sensor and the crankshaft signal plate. However, the malfunction still recurs when the engine reaches operating temperature: the tachometer occasionally stops working on the instrument cluster, and the same fault code reappears. At this stage, the repair work reached an impasse.
To verify this hypothesis, the technicians took a multimeter, selected an appropriate measuring range, moved close to the ignition coil, and performed a test to check for interference sources near the coil.
The test result confirmed that the ignition coil was indeed generating electrical power interference.
At this point, the faulty component was identified as the ignition coil.
After removing the ignition coil, technicians found that it was not an original factory part, and the part number of the replacement coil differed from the original one, as shown in the figure.
Case Summary
This vehicle fault was caused by a non-OEM ignition coil. After long-term operation, high-voltage electricity generated by the coil leaked into the Engine Control Module (ECM), interfering with the supply voltage of the crankshaft position sensor and resulting in abnormal sensor operation.
This type of fault is relatively rare yet extremely difficult to troubleshoot. This troubleshooting case indicates that technicians should not judge faults merely based on superficial phenomena in future repair work. Blindly replacing spare parts is not advisable, and interference signals must never be overlooked.
